How can I move my "last name" text box up alongside the "first name" text box?

Please take a look a this guide: http://www.jotform.com/help/90-Form-Field-Positioning, It will help you to accomplish what you need:

Here is the link to my cloned version of your form, you may clone it if you want to, let us know if you need further help, we will be glad to assist you.